1. Field of the Invention
This invention relates to a unique siloxane composition which will cure to form a dielectric silicone gel. More specifically, the disclosed composition is especially formulated so that it will cure readily at ambient or elevated temperatures to form a silicone gel which can withstand very low temperatures (ca. -120.degree. C.) without a detrimental effect on the physical stability of the gel and its low temperature properties.
2. Description of the Prior Art
The principal use intended for the gels formulated according to the present invention is as a dielectric potting gel. The use of silicone potting compounds to encapsulate electrical assemblies is well documented. U.S. Pat. No. 3,436,366, issued Apr. 1, 1969, to Modic describes how silicone materials are used to provide electrical insulation and various degrees of protection from thermal and mechanical abuse. The Modic patent also describes the possibility of formulating a dielectric potting compound using a polyorganosiloxane having only methyl and vinyl substituents. However, those substituents are arranged in such a way so as to promote a great deal of crosslinking resulting in significant tear strength. In particular, Modic teaches using an organopolysiloxane having methylvinylsiloxane and SiO.sub.2 units. Both of those units promote a high degree of crosslinking resulting in a product having very little in common with the supple gel of the present invention. Furthermore, the Modic material does not have the unique low temperature property exhibited by the gels of the present invention.
U.S. Pat. No. 3,020,260, issued Feb. 6, 1962, to Nelson is perhaps more relevant to the present invention. In Example 10 of that reference, a silicone gel formulated from siloxane copolymers having only methyl and vinyl substituents is described.
More precisely, the Nelson patent teaches a method of formulating a silicone potting gel by reacting a blend of 86.95 parts by weight of copolymer (I) consisting of Me.sub.2 SiO, MeViSiO and Me.sub.3 SiO.sub..5 and 8.27 parts by weight of copolymer (II) consisting of Me.sub.2 SiO, MeSiO.sub.1.5 and Me.sub.3 SiO.sub..5 with 4.78 parts by weight of an organohydrogensiloxane crosslinker in the presence of a platinum catalyst. The symbol "Me" represents a "--CH.sub.3 " radical and the symbol "Vi" represents a "--CH.dbd.CH.sub.2 " radical.
The silicone potting gel resulting from the Nelson formulation will form a dielectric silicone gel, but because of the ratios of the units employed and the presence of a unit having an internal vinyl function (MeViSiO), it will not have the structural properties and, hence, will not exhibit low-temperature stability of the siloxane gel prepared according to the present invention.
Omietanski in U.S. Pat. No. 3,361,714, issued Jan. 2, 1968, discloses certain "Comb" polymers and "Star" polymers having low temperature properties. These polymers are non-random polymers and have a generic formula ##STR1## wherein R and R' are monovalent hydrocarbyl radicals, R.sup.2 is a monovalent hydrocarbyl radical or substituted monovalent hydrocarbyl radical or substituted monovalent hydrocarbyl radical, n is 0 to 25, m is 1 to 21, w is greater than zero, Z' is hydrogen or R.sub.3 Si--, and Z is hydroxyl or R.sub.3 SiO--. Omietanski teaches that fluids and elastomers cured with organic peroxides can be obtained which exhibit low temperature properties.
Mindful of the properties and limitations of the silicone potting compounds known and described in the prior art, we endeavored to formulate a siloxane gel from a siloxane polymer having only methyl and vinyl substituents. However, these well-known units have been combined in such a manner so as to provide a novel polymeric composition which will cure rapidly to a gel at ambient or elevated temperatures and maintain its gel properties at very low temperatures.